Summary:
Fixes T5677.
- Instead of using `sequence == 0` to mean "this is the backlog column", flag the column explicitly.
- Migrate existing sequence 0 columns to have the flag.
- Add the flag when initializing or copying a board.
- Remove special backlog logic when reordering columns.
Test Plan:
- Migrated columns, viewed some boards, they looked identical.
- Reordered the backlog column a bunch of times (first, last, middle, dragged other stuff around).
- Added tasks to a project, saw them show up in the reordered backlog.
- Initialized a new board and saw a backlog column show up.
- Copied an existing board and saw the backlog column come over.
- Tried to hide a backlog column.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T5677
Differential Revision: https://secure.phabricator.com/D10189
